This invention relates to vehicle detector systems of the type employing period shift measurement.
Vehicle detector systems are known which employ the principles of period shift measurement in order to determine the presence of a vehicle in or adjacent to an inductive loop mounted on or in a roadway. In such systems, a first oscillator, which typically operates in the range from about 20 to about 50 Khz, is used to produce a periodic signal in a vehicle detector loop. A second oscillator operating at a much higher frequency is commonly used to generate a sample count signal over a fixed number of loop cycles. The relatively high frequency count signal is typically used to increment a counter which stores a number corresponding to the sample count at the end of the fixed number of loop cycles. This sample count is compared with a reference count representative of a previous count in order to determine whether a vehicle has entered or departed the region of the loop.
The initial reference value is obtained from a sample count and stored in a reference counter. Thereafter, successive sample counts are obtained on a periodic basis, and compared with the reference count. If the two values are essentially equal, the condition of the loop remains unchanged, i.e., a vehicle has not entered the loop. However, if the two numbers differ by at least a threshold amount in a first direction (termed the Call direction), the condition of the loop has changed and may signify that a vehicle has entered the loop. More specifically, in a system in which the sample count has decreased and the sample count has a numerical value less than the reference count by at least a threshold magnitude this change signifies that the period of the loop signal has decreased (since fewer counts were accumulated during the fixed number of loop cycles), which in turn indicates that the frequency of the loop signal has increased, usually due to the presence of the vehicle in or near the loop. When these conditions exist, the vehicle detector generates a signal termed a call signal indicating the presence of a vehicle in the loop.
Correspondingly, if the difference between a sample count and the reference count is greater than a second threshold amount, (i.e., the sample count plus 5 counts is the second threshold amount is larger than the reference count), this condition indicates that a vehicle which was formerly located in or near the loop has left the vicinity. When this condition obtains, a previously generated call signal is dropped.
The call signals are used in a wide variety of applications, including vehicle counting along a roadway or through a parking entrance or exit, vehicle speed between preselected points along a roadway, vehicle presence at an intersection controlled by a traffic control light system or in a parking stall, and numerous other applications. In all applications, it is necessary to periodically update the reference value so that the vehicle detector system can be dynamically adjusted to varying conditions. For example, the loop wire, connecting cables and associated electronic analog circuitry are typically subject to widely varying temperature conditions, which cause the frequency of the loop signal to vary in a somewhat unpredictable manner. If the loop frequency drifts between sample periods by an amount equivalent to the period threshold count in the Call direction, a false call will be detected (since the sample count will be less than the reference count by the threshold value), even though no vehicle has actually entered the loop. This false call will be manifested by a green light in the lane controlled by the detector issuing the false call, even though no vehicle is present in that lane. This is clearly highly undesirable as it adversely affects vehicle flow through a controlled system.
In the past, the problem of loop frequency drift has been addressed by a number of techniques. According to one known technique, the reference is slowly adjusted (typically once every 2 seconds) after taking the sample count by examining the difference between the sample count and the reference and (a) decrementing the reference by one count when the sample count is less than the reference and (b) incrementing the reference by one count whenever the sample count exceeds the reference. This technique suffers from several disadvantages. Firstly, while the slow tracking of the loop drift afforded by this approach from the No Call to Call direction is desirable, it is highly undesirable in the opposite direction (i.e., the Call to No Call direction). This is principally due to the fact that, starting with the Call condition the reference is decremented to an artificially low value (typically 100 counts or more below the previous No Call reference value). If the vehicle which generated the call leaves the loop and another vehicle enters the loop, this new Call condition will not be detected, since the new sample count will not be less than the current reference value until the reference is incremented by the testing threshold amount (which would take many cycles). As a result, the newly entered vehicle will not be serviced by the traffic control system (i.e., issuance of a green).
In an attempt to avoid this disadvantage, a modification of this first technique has been developed which decrements the reference (typically once every 2 seconds) when the sample count is less than the reference value (the same as the decrementing in the first technique), but which changes the reference to the sample count whenever the sample count exceeds the current reference value. This technique introduces another disadvantage. Specifically, when a noise pulse is generated in the loop which causes the sample count to erroneously rise in value by a significant amount, which is a common occurrence, the new reference value is incorrectly set to an artificially high value. When the noise disappears (typically before the next sample count is taken), the new sample count drops back to the nominal No Call value, which causes a false call to be registered, with the observable disadvantages noted above. Further, since the reference is only decremented (typically once every 2 seconds) , it may take a long period of time (possible hours) for the reference to be readjusted to the nominal No Call value. During this period of adjustment, false calls are registered for each successive sample, and false greens are issued for the same period of time, which totally disrupts the traffic control system.
Still further compounding this problem is the fact that an intermittent open loop can also disrupt the reference adjustment process by suddenly raising the loop inductance, which causes a corresponding increase in the sample count. For the case of a shorted loop, the reference value is gradually decremented to the extremely small value of the sample count registered by the shorted loop during which time a false call will be registered. If the short self-corrects with a vehicle in the loop, the next sample count will exceed that of the invalid reference value and no call will be detected. The new reference will then be adjusted to the sample count obtained with the vehicle. However, since no call will be generated so long as the vehicle remains in the loop, the vehicle will never obtain a green signal, which is highly undesirable.
As noted above, in order to register a call from the No Call condition, the count sample must be smaller than the reference value by a threshold amount. This threshold amount is necessary in order to avoid jitter around zero and sample count changes due to vibration of vehicles in or adjacent the loop, which can cause slight changes in the sample count value. In order to avoid these two effects, vehicle detector systems have been designed with fixed hysteresis for the Call/No Call conditions. In one popular system, two thresholds have been employed: A first threshold of 8 counts between the reference value and the sample count in the No Call to Call direction, and a second value of 5 counts in the Call to No Call direction. Specifically, in order to register a call the difference between the reference and the sample count must be at least 8; while to register a No Call from a Call condition the difference between the reference value and the sample count must be at least 5. While fixed hysteresis has been found useful, it suffers from the disadvantage that different applications optimally require different hysteresis values. For example, in traffic intersection control applications, the 8, 5 fixed hysteresis values function well. However, for parking applications in which the vehicle traffic moves quite slowly a vibrating metal part on a vehicle (e.g, the bumper) causes vibration changes to the sample count which are greater than the three count difference in the 8, 5 fixed hysteresis system. Consequently, for such applications greater hysteresis must be used, such as a difference of at least 12 for a call to be registered and a difference of five or less for the call to be extinguished. As a result of these differing hysteresis requirements, systems in the past have been specially designed for specific applications with fixed hysteresis, which requires a large number of different models of the same basic vehicle detector in order to meet consumer demands.